A known technique for tracking items is to mount a radio frequency identification (RFID) tag on each item to be tracked. These RFID tags transmit wireless signals, and readers are provided to receive these signals. The power levels of these wireless signals are limited by law. Consequently, in existing systems, the effective range of these wireless signals is typically only about 300 feet.
One practical example of a tracking application is that items of military equipment are often tracked using RFID tags. However, in a military theater of operations, many of the items to be tracked may be on the enemy's side of a battle line, where it is impractical to install and/or maintain an array of multiple readers that can reliably read RFID tags.
An example of a non-military application is a shipping port. In both military and non-military applications, it is typically necessary to provide an array of readers to cover a given area, and such an array contains a large number of readers. Given the number of readers, the cost of installing these types of systems is relatively high.
A further consideration that it is desirable to be able to reliably detect movement and/or transport of an item such as a shipping container. One known technique is to provide a motion sensor on the shipping container. However, a motion sensor cannot differentiate between movement of the item within a monitored area, and movement of the item out of the monitored area, for example due to theft.
Still another consideration is that RFID tags almost always run on battery power. Consequently, it is always desirable to conserve a tag's battery power, in order to maximize the length of time from insertion of a newly-charged battery until the battery becomes too discharged to properly operate the tag.